1. Field of the Invention
The present invention relates to the employment of multipolymers of vinylidene chloride as thin coatings for oriented polypropylene packaging films. Such coatings increase the resistance of polypropylene films to the transmission of gases and vapors thereby enhancing the films' utility when packaging products which may be oxygen or moisture sensitive. Additionally, the present invention provides a primer coating for such polypropylene films whereby the adhesion of the vinylidene chloride multipolymer coating to the films is improved.
2. Brief Description of the Prior Art
The employment of multipolymers of vinylidene chloride as thin coatings for oriented thermoplastic films such as polypropylene film has been recognized in the prior art as an effective means of increasing the heat sealability of such films and additionally increasing the resistance of the films to the transmission of gases and vapors. This latter characteristic is particularly useful when such films are employed to package products which are sensitive to, or attacked by, oxygen or moisture.
Of particular importance when such thermoplastic film substrates are coated with these vinylidene chloride multipolymer compositions is to insure that adhesion of the coating layer to the polypropylene substrate is adequate. For example, in many packaging applications, it is necessary for the coated film to be heat sealed either to itself or to other films to form a tightly closed package. If the coating adhesion to the base film is inadequate, the packages are liable to prematurely open when subjected to stress.
In general, vinylidene chloride base multipolymer compositions, containing at least 50% vinylidene chloride by weight, do not adhere well to polypropylene film surfaces, even when the latter have been subjected to well known pretreatment operations such as, for example, treatment by corona discharge, flame, or oxidizing chemicals. Accordingly, it has been found necessary to apply a thin intermediate primer layer to the surface of the polypropylene base film before applying the vinylidene chloride multipolymer topcoat generally in the form of an aqueous dispersion or latex.
Primers which have been used in the past include solutions of polyurethane resins dissolved in organic solvents. See, for example, U.S. Pat. No. 3,023,126. Such materials have been found to serve the desired function, i.e., insure adequate adhesion between the base film and the top coating. However, primer compositions based upon polyurethane require solvents to dissolve the urethane polymers and such solvents pose problems. For example, they are expensive, flammable, frequently toxic, and would lead to environmental contamination if discharged into the atmosphere. Alternatively, solvent recovery systems are quite expensive. Furthermore, polyurethane resin solutions are extremely sensitive to moisture and require great care to insure exclusion of water during storage and use.